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People often advise doing  just big and heavy lifts like pull-ups if you want bigger biceps, and heavy dips if you want bigger triceps. Many even recommend heavy lifts like back squats. They say to forget about direct (isolated) work, like biceps curls and triceps press-downs.
This advice is based on the anabolic hormonal responses triggered by big lifts — specifically that of endogenous testosterone.
While hormones do contribute to muscular hypertrophy, they alone don’t trigger maximum peripheral growth.
During lifting, muscular recruitment patterns can differ significantly from person to person. For example, EMG studies show that two people performing the same exercise may activate muscles to very differing degrees, or may recruit different muscle groups entirely.
Perhaps the advice to use just the big lifts comes from people who are able to activate a larger percentage of arm muscles during pull-ups or dips. This is certainly not the case in all people.
If you’re after the aesthetics of muscular arms, you should probably include direct arm-work. This is because muscle growth relies more on just hormones; it depends also (if not more so) on direct mechanical stress.
As an example, several years ago my friend broke his arm, but continued doing big lifts like back squats, one-arm pull-ups (assisted), and dips (also assisted). He also continued doing direct arm-work with his healthy arm, while his other was mostly rested in a sling.
After 8 weeks of this, you could see that his rested arm was significantly atrophied. His good arm retained equal muscle mass, if not slightly bigger. And this atrophy was while still doing the big lifts.
So, the big lifts are great, but for many people seeking more muscular arms, direct arm work should be included.
I’m all for functional strength training, but who wouldn’t want great-looking muscular arms?